1. Field of The Invention
This invention relates to an impulsive seismic energy source for use within deep bore holes and more particularly to pressurized gas discharging apparatus capable of insertion into narrow boreholes for generating seismic impulses therein through sudden discharge of high pressure gas.
The desirability for generating abrupt seismic impulses deep within a narrow borehole exists in seismic surveying and similar applications. For seismic surveying, powerful impulses are generated underground in boreholes and are utilized to investigate subsurface geological conditions and formations. Other applications for the use of sudden release of pressurized gas to generate abrupt powerful impulses within a borehold may include testing and signalling purposes as well as additional practical applications which may become more fully developed as the technology in this area advances.
It is expensive to drill boreholes in the ground for seismic surveying and the cost of drilling and the cost of the tooling increases rapidly for increasing borehole sizes above 3 inches in diameter. Therefore, it is desirable from a cost viewpoint to keep the diameter of such boreholes at 3 inches in diameter.
As used herein the term "pressurized gas" is intended to include any pressurized gas of pressurized mixture of gases which can be used in an airgun, for example such as compressed air, compressed carbon dioxide, compressed nitrogen, pressurized steam, and the like. In most cases it is easier and cheaper to use compressed air, supplied from a mobile compressor or from portable tanks, and air is usually the preferred pressurized gas to be employed. However, in certain cases, for example if the borehole is suspected to contain combustible vapors or inflammable gas, then it may be desirable to use a non-combustion-supporting gas, such as carbon dioxide or nitrogen. Also "pressurized" is intended to mean at an elevated pressure substantially above atmospheric pressure, for example, such as in the preferred range from 500 pounds per square inch (p.s.i.) up to 4,000 p.s.i., although higher of lower pressures of the pressurized gas may occasionally be used.
2. Description Of The Prior Art
Pressurized gas discharging apparatus, called an "airgun" for use in seismic surveying under water, including marsh land, swamp or mud areas is disclosed in my U.S. Pat. No. 3,808.822. A process and system for increasing the load bearing capacity of soil using pressurized gas discharging apparatus inserted in a hollow tubular member positioned in the soil and repeatedly abruptly releasing the pressurized air near the lower end of the member to produce powerful impulses for repeatedly impelling material, such as sand, gravel, concrete or the like which has been fed down the hollow member, outwardly into the soil to create a load bearing column of the material is disclosed in U.S. Pat. Nos. 3,707,848 and 3,793,844. Please see also U.S. Pat. No. 3,808,823 relating to apparatus for use in this process and system. The pressurized gas discharging apparatus described in the aforementioned patents have been referred to as airguns and the airgun is "fired" when the highly pressurized air of other gas it contains is caused to be abruptly released and discharged into the surrounding environment.
The airguns known prior to the present invention generally include, as operative components, charge chamber means for holding gas under high pressure, a release means which can be actuated to abruptly release the pressurized gas through discharge ports provided in the walls of the charge chamber, means for supplying gas under high pressure to the charge chamber means, and control means for causing the release means to be actuated to release the pressurized gas. The release means generally comprises a shuttle including a first (or releasing) piston positioned in the charge chamber for retaining the pressurized gas therein and a second (or controlling) piston positioned in an operating cylinder or control chamber in which pressurized gas acts against the shuttle piston to maintain it in "closed" position until it is to be released. The shuttle pistons are interconnected by a shaft which is reciprocally positioned between the operating cylinder and the charge chamber and is hollow for permitting a supply of pressurized gas to flow first to the operating cylinder and thence to the charge chamber.
The airguns may be self-firing by appropriate relative sizing of the exposed surface areas of the shuttle pistons or may be fired by operation of a solenoid valve for controlling flow of pressurized gas through various firing passages to cause the shuttle to be abruptly operated. A suitable solenoid valve means is described in U.S. Pat. No. 3,588,039. Both such modes of airgun operation are described in my U.S. Pat. No. 3,379,273 and the reader is referred thereto for a more detailed description of the airgun. More information about airguns may also be had by reference to my U.S. Pat. Nos. 3,249,177 and 3,653,460.
The operation of airguns within boreholes in soil involves special environmental consideration including the presence of potentially harmful surrounding materials such as dirt, grit and the like. In addition, economically drillable boreholes are generally of a narrow diameter, for example, as small as three inches, and pressurized gas discharging apparatus which is to be inserted therein is desirably of an outside diameter and shape which enables trouble-free insertion within such borehole. Thus, the airgun disclosed for the use described in U.S. Pat. Nos. 3,707,848 and 3,793,844 can enter a hollow pipe member having a diameter from about 6 inches up to 5 feet, and the airgun for use in underwater seismic surveying may have as large a diameter as is convenient for towing through the water, for example, over 5 inches to contain a high volume charge. None of the airguns shown in any of my above patents can fit within narrow economically drillable boreholes of 3 inches in diameter.